A North–South genetic divide in marine Gastrotricha: distinct Mediterranean lineages across four species

The aim of this study is to test whether four marine gastrotrich species shared between the North Sea and the Mediterranean Sea represent single, pan-European taxa or instead comprise hidden, genetically structured lineages. Using single-specimen whole-genome sequencing, multigene phylogenetics (18S, 28S, COI), and multiple species-delimitation methods, we analyzed populations of Xenotrichula intermedia , Halichaetonotus aculifer , H. atlanticus , and Turbanella bocqueti sampled from the North Sea and the Mediterranean Sea. All four investigated species were resolved into at least two phylogenetic lineages, consistently separating North Sea from Mediterranean populations. In X. intermedia , Mediterranean specimens further split into distinct Ligurian and Adriatic lineages, revealing deep cryptic diversity despite morphological uniformity. Distance-based and coalescent-based delimitation approaches strongly corroborated these divisions, supporting the existence of pronounced genetic divergence and geographic structuring across the investigated populations. Additional internal structuring was detected within northern populations of both H. atlanticus and T. bocqueti , with H. atlanticus showing a shallow but still detectable North Sea–Mediterranean split. Overall, our results demonstrate a consistent genetic differentiation between North Sea and Mediterranean populations and uncover substantial hidden diversity in the European marine gastrotrichs of the examined areas, challenging assumptions of broad dispersal and highlighting the need for integrative taxonomic reassessment of meiofaunal biogeography.